It is generally difficult, although desirable, to provide an apparatus for selectively control oxygen concentration and steam in the atmosphere gas within a food preserving container, indoors or the like. There remains room undeveloped as a method of keeping boiled rice although the art of cooking the rice is considerably improved. For example, the rice cooker for cooking the rice and keeping it warm is developed in recent years in rice cooking.cndot.warming mechanism applying Neuro Fuzzy theory so that rice can be cooked as tasty as possible and thereafter, the cooked rice can be kept in a tasty condition. The rice can be cooked tasty, kept tasty by the warming pot by the application of the induction heater (I.multidot.H). The cooked rice enters into a warmth keeping condition. Thereafter, the cooked rice is taken cut after the cover has been opened, and the remaining rice is retained again in a warm keeping condition. When the cooked rice is taken out after several hours, the rice becomes often yellow in tint, causing the unplesant smells. The cooked rice becomes untasty because of oxidization caused in it although in the keeping condition.
A direct oxidizing prevention of the cooked rice is not realized although a method of keeping the cooked rice in a condition as good as possible by the adjustment of the warmth keeping temperature is considered. A means for removing the oxygen of the warmth keeping pot is proposed in Japanese Patent Application (unexamined) No. 5-154039 as shown in FIG. 9 wherein 1 denotes an rice boiler, 32 a lid, 33 an inner pot, 34 an angle, 35 a heater, 38 an inner lid, 39 an intake pipe, 40 an opening, 41 a valve, 42 a separating film, 43 an opening and 44 a vacuum device. The means is composed of gas separating films through which only oxygen is transmitted, and a pressure reducing apparatus (mechanical pump). A method of discharging the oxygen existing within the warmer pot out of the warmer is proposed. However, the rice cooker proposed above has the following problems. A valve apparatus, gas separating films, a pressure reducing apparatus are provided within the cover body, resulting in bigger cover body, increasing the weight thereof, thus interfering with the opening, closing operations of the cover body. There are problems such as performance deterioration due to dewing on the gas separating films, the operation noises of the pressure reducing apparatus and so on. Further, it is difficult to put the apparatuses into practical use without provision of a means for taking measures with respect to the pressure reducing condition in the case of the oxygen discharging. Also, although an oxygen pump using a perovskite type of oxide is proposed in Japanese Laid-Open Patent Application Tokkaisho No. 56-134502, a system to be driven by optical application is not much practical.